1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a liquid crystal display (LCD) device having a cholesteric liquid crystal (CLC) color filter and a method for manufacturing the same.
2. Discussion of the Related Art
As the society has been progressing to an age of information, a flat panel display device having a superior qualities such as a small thickness, a low weigh and a low power consumption has been requested. Accordingly, liquid crystal display devices have been most actively applied to many modem conveniences such as notebook computers and desk top computers because of their superior resolution, improved color images display and high quality displayed images.
Generally, the liquid crystal display (LCD) device has upper and lower substrates and a liquid crystal layer between the upper and lower substrates. Each substrate has an electrode for forming an electric field between upper and lower substrates. The liquid crystal display (LCD) device display images by controlling a light transmissivity depending on an alignment of liquid crystal molecules. The light transmissivity can be controlled by aligning the liquid crystal molecules by forming an electric field between the electrodes of the upper and lower substrates. A structure of a typical liquid crystal display (LCD) device will be described more in detail hereinafter with reference to FIG. 1.
FIG. 1 illustrates a cross-sectional view of a typical liquid crystal display (LCD) device.
Referring to FIG. 1, the liquid crystal display (LCD) device usually has first and second substrates 10 and 20. The first substrate 10 has a thin film transistor “T1” that has a gate electrode 11, a source electrode 15a, a drain electrode 15b, an active layer 13 and an ohmic contact layer 14. A gate insulating layer 12 is formed over the gate electrode 11 and the first substrate 10. A passivation layer 16 is formed over the thin film transistor “T1” and has a drain contact hole 16c exposing a portion of the drain electrode 15b. A pixel electrode 17 is formed on the passivation layer 16 and connected to the drain electrode 15b via the drain contact hole 16c. A black matrix 21 is formed beneath the second substrate 20 corresponding to the thin film transistor “T1” and color filters 22a and 22b are formed thereunder. A common electrode 23 is formed beneath the color filters 22a and 22b with transparent conductive metal material. Each of the color filters 22a and 22b corresponds to each of the pixel electrodes 17. A liquid crystal layer 30 is disposed between the common and pixel electrodes 23 and 17. An alignment of liquid crystal molecules of the liquid crystal layer 30 is changed by an electric field that is formed between the common and pixel electrode 23 and 17 by applying voltage to the common and pixel electrodes 23 and 17. Though not shown in FIG. 1, an alignment layer is further formed on the pixel electrode 17 and beneath the common electrode 23 to initially align the liquid crystal molecules. First and second polarizers 41 and 42 are formed respectively beneath the first substrate 10 and on the second substrate 20. The polarizers 41 and 42 converts a natural light into a linearly polarized light by transmitting only a natural light component parallel with a light transmission axis. The light transmission axis of the first polarizer 41 forms an angle of 90° (degree) with the light transmission axis of the second polarizer 42. By the way, because the aforementioned liquid crystal display (LCD) device cannot emit light for itself, an additional light source is requested. The liquid crystal display device can be divided into two different types depending on a position of the light source. One is a transmissive liquid crystal display (LCD) device, and the other is a reflective liquid crystal display (LCD) device.
The transmissive liquid crystal display (LCD) device has the light source behind a liquid crystal panel (e.g., under the first polarizer 41 of the first substrate 10 in FIG. 1) and displays images by irradiating incident light from the light source to the liquid crystal. Accordingly, the common and pixel electrodes 23 and 17 must be formed of transparent conductive material and the first and second substrates 10 and 20 must also be formed of transparent material.
Meanwhile, the reflective liquid crystal display (LCD) device controls the transmissivity according to the alignment of the liquid crystal molecules by reflecting the ambient light or the artificial light from an exterior of reflective liquid crystal display (LCD) device. In the reflective liquid crystal display (LCD) device, the pixel electrode 17 is formed of conductive material that has a superior reflective properties, and the common electrode 23 is formed of transparent conductive material to transmit the ambient light. The reflective liquid crystal display (LCD) device does not need the first polarizer 41 and the first substrate 10 may be formed of material having a low transmissivity or opaque material.
Because the transmissive liquid crystal display (LCD) device utilizes an artificial light source such as a backlight, it can display images even in dark environments. Because the reflective liquid crystal display (LCD) device utilizes an ambient light as the light source, it has a low power consumption. The usual color filter for the aforementioned liquid crystal display (LCD) device is an absorption type color filter, and thus a lot of light loss occurs as the light transmits through the color filter, resulting in a decrease of a luminance of the liquid crystal display (LCD) device. Accordingly, the LCD device having a cholesteric liquid crystal (CLC) color filter, which utilizes the property of the CLC, has been researched and developed in the field. If the CLC color filter is used for the transmissive LCD device, the luminance can be improved compared to LCD devices having the absorption type color filter. If the CLC color filter is used for the reflective LCD device, color reproducibility and contrast ratio can be improved compared to LCD deviceS having the absorption type color filter.
The CLC color filter is formed using a selective reflection property of the CLC. The CLC color filter has a function of a mirror when each liquid crystal layer having a helical structure forms a perfect alignment. That is, if all helical axes of the CLC align vertically to the substrate, the CLC color filter reflects the incident light at a surface of the CLC color filter in a mirror reflection in which an incidence angle and a reflection angle are same. The CLC color filter does not reflect all incident light, but reflects the light in a particular wavelength range depending on a helical pitch. Accordingly, red (R), green (G) and blue (B) colors can be displayed by locally controlling the helical pitch of a portion of the CLC color filter.
A rotational direction of the CLC helix is important characteristic in the helical structure of the CLC itself. The rotational direction of the CLC helix is an important factor for the polarization phenomenon. That is, the direction of a circular polarization of the reflected light depends on whether the helix structure of the CLC is right-handed or left-handed. The right-handed CLC reflects light having a right circular polarization that has a wavelength corresponding to the pitch of the right-handed CLC. Because the ambient light is a mixture of light having a right circular polarization and light having a left circular polarization, light having either the right circular polarization or the left circular polarization can be extracted according to the structure of the CLC, i.e., a right handed helix or left-handed helix. Because polarization property, i.e., a linear polarization, is used in the conventional liquid crystal display devices, the degree of light utilization will be greatly improved using the CLC, and will result in an effective reduction of power consumption compared to the color filters including pigment or dye.
FIG. 2 illustrates a cross-sectional view of a transmissive liquid crystal display (LCD) device having a CLC color filter according to the related art.
Because the liquid crystal display (LCD) device having the CLC color filter in FIG. 2 has a same structure as that of FIG. 1, an explanation on same elements will not be described again for the sake of a convenience. Referring to FIG. 2, first and second substrates 50 and 60 are spaced apart from each other and opposing each other. A thin film transistor “T2” and a pixel electrode 57 are formed on the first substrate 50 and a black matrix 61, cholesteric liquid crystal (CLC) color filters 62a, 62b, 62c and 62d and a common electrode 63 are formed on the second substrate 60. The CLC color filter has a double layer structure, each layer of which reflects light in a different wavelength range. Though not shown in FIG. 2, an alignment layer may further be formed between the CLC color filters 62a and 62c and the black matrix 61 to initially align CLC molecules. Because the LCD device of FIG. 2 is a transmissive type LCD device, a backlight (not shown) is disposed over the first substrate 50 and irradiates light to the second substrate 60. Because the CLC reflects light in a particular wavelength range depending on the helical pitch of the CLC, as mentioned before, the light in a wavelength range other than that of a desired color must be reflected on the surface of the CLC color filter. If the red color is to be displayed, one of the double layers of the CLC color filter must reflect one of the green color and the blue color, and then the other layer of the CLC color filter must reflect the remaining color. That is, a first layer of the CLC color filter 62b, for example, reflects a component of the incident light in a wavelength range of the blue color and transmits the remaining components of the incident light. The incident light components that passed through the first layer of the CLC color filter 62b reach the second layer of the CLC color filter 62a, for example. The second layer 62a of the CLC color filter reflects the light in a wavelength range of the green color. Accordingly, only the light in a wavelength range of the red color can be transmitted and thus the red color can be displayed as a result.
FIG. 3 illustrates a cross-sectional view of a reflective liquid crystal display (LCD) device having a CLC color filter according to the related art.
Referring to FIG. 3, description of a thin film transistor “T3” is simplified because the thin film transistor “T3” has a same structure as that of FIG. 1 and FIG. 2. A light absorption layer 72 is formed on a first substrate 71, and then cholesteric liquid crystal (CLC) color filters 73a, 73b and 73c are formed on the light absorption layer 72. The CLC color filters 73a, 73b and 73c respectively display the red (R), the green (G) and the blue (B) colors by reflecting light in a wavelength range of the red, the green or the blue color corresponding to each portion of the CLC color filters 73a, 73b and 73c. A common electrode 74 is formed of transparent conductive material on the CLC color filters 73a, 73b and 73c. Because the CLC color filters 73a, 73b and 73c serve as a reflector as well as a color filter, an additional reflector is not required. A second substrate 75 opposing the first substrate 71 is spaced apart from the first substrate 71. The thin film transistor “T3” and a transparent pixel electrode 76 are formed beneath the second substrate 75. A liquid crystal layer 77 is disposed between the first and second substrates 71 and 75. A polarizer 78 is further formed on the second substrate 75. Though not shown in FIG. 3, an alignment layer is further formed respectively on the absorption layer 72 and the common electrode 74 and beneath the pixel electrode 76. In addition, a retardation layer (not shown), called a quarter wave plate, having a phase difference of λ/4, may further be formed between the polarizer 78 and the second substrate 75.
FIG. 4 illustrates a laminated structure near a seal pattern area of a reflective liquid crystal display (LCD) device having a CLC color filter according to a first example of the related art.
Referring to FIG. 4, an inorganic insulating layer 92, a organic insulating layer 94 and a chromium (Cr) layer 96 are sequentially formed beneath an upper substrate 90. An absorption layer 82, an alignment layer 84, a CLC color filter 86 and a transparent electrode 88 are sequentially formed on a lower substrate 80. Additional alignment layers 98 and 89 are formed respectively beneath the chromium (Cr) layer 96 and on the transparent electrode 88 to initially align liquid crystal molecules. A liquid crystal layer 85 is disposed between the upper and lower substrates 90 and 80 and the upper and lower substrates 90 and 80 are attached by a seal pattern 99 that is formed on one of the upper and lower substrates 90 and 80. The seal pattern 99 maintains a cell gap between the upper and lower substrates 90 and 80 and prevents the liquid crystal from leaking out. The upper substrate 90 of the liquid crystal display (LCD) device in FIG. 4 further has another transparent electrode (not shown) that forms an electric field with the transparent electrode 88 of the lower substrate 80. Referring to FIG. 4, the transparent electrode 88 is formed on a whole area of the lower substrate 80. Accordingly, the liquid crystal layer 85 between the upper and lower substrates 90 and 80 can be protected from being contaminated by the CLC color filter 86 on the lower substrate 80, as shown in “A” of FIG. 4. However, because the adhesive property between the CLC color filter 86 and the transparent electrode 88 that is formed of indium tin oxide (ITO) is not good, an adhesive strength of the seal pattern 99 becomes weaker than that of a standard structure in FIG. 5. Thus seal pattern may be broken after the upper and lower substrates 90 and 80 are attached. However, the aforementioned problem may be overcome by improving the adhesive property between the CLC color filter 86 and the indium tin oxide (ITO) electrode 88 by improving a physical property of the CLC color filter 86.
FIG. 5 illustrates a laminated structure near a seal pattern area of a reflective liquid crystal display (LCD) device having a CLC color filter according to a second example of the related art.
Referring to FIG. 5, the basic structure of the LCD device according to the second example of the related art is same as in FIG. 4 except the indium tin oxide (ITO) electrode 188 on a lower substrate 180. As shown in FIG. 5, the indium tin oxide (ITO) electrode 186 is not formed on the whole area of the lower substrate 180, but formed spaced apart from a seal pattern 199 as illustrated as “B”. This structure of the LCD device is obtained by the standard manufacturing process. Though the adhesive strength of the seal pattern 199 can be improved with this structure, there exists an area where the CLC color filter 186 contacts the liquid crystal layer 185. Accordingly, because the CLC color filter 186 and the liquid crystal of the liquid crystal layer 185 are the same kind of material, contact of the CLC color filter 186 with the liquid crystal of the liquid crystal layer 185 may cause a swelling phenomenon of the liquid crystal and an interaction force. Above all, the cholesteric liquid crystal (CLC) color filter 186 may contaminate the liquid crystal layer 185 because of contaminants such as ions and particles. Accordingly, the contamination of the liquid crystal causes a bad action, such as a stain in the liquid crystal layer, and thus a quality of displayed images may be deteriorated.